Such a steady erosion of customers could spell doom for any industry. But Big Tobacco is counteracting this persistent trend by recruiting replacement smokers - primarily from the ranks of American youths.

Alarmed at such targeting of young people, officials in Massachusetts enacted a ban on outdoor advertisements promoting tobacco products anywhere within 1,000 feet of a public playground or school.

The action touched off a legal dispute between the two sides and set the stage for what could be the most important First Amendment case in years. The dispute, which comes before the US Supreme Court today, pits the value of censorship in certain circumstances against the value of upholding the free exchange of ideas without regard to content or popularity.

Today, the justices will hear oral arguments in two consolidated cases filed by tobacco companies challenging the Massachusetts advertising restrictions as a violation of their free-speech right to promote their products.

Massachusetts officials say the restrictions are a reasonable use of state power to protect children from what the officials view as predatory product promotion.

"Because tobacco advertising exerts an 'undue influence' on children - who constitute the vast majority of new tobacco users - it can be singled out for regulation," writes William Porter in a brief filed on behalf of Massachusetts Attorney General Thomas Reilly.

Lawyers for the tobacco companies counter that the advertising restrictions amount to unconstitutional government censorship of a form of disfavored speech. "Because Massachusetts' regulations are directed not at any commercial aspect of the prohibited advertising but at the ideas conveyed, the regulations strike at the core of First Amendment values," writes Jeffrey Sutton in his brief for the tobacco companies.

Massachusetts isn't alone in enacting such tobacco advertising restrictions. California, Utah, Texas, Kentucky, and Arizona have passed similar measures, and many other states are considering regulations.

Some analysts say tobacco advertisements are an obvious and justifiable target for government restrictions. But others stress that the First Amendment is designed to protect all speech, particularly messages that are unpopular or controversial.

"It is easy to defend apple pie, but that is not what the First Amendment is about," says Jeff Perlman of the American Advertising Federation in Washington, which filed a friend-of-the-court brief in the case. "The First Amendment is to protect the underdog, to protect disfavored speech," Mr. Perlman says.

Some legal experts say the case may prompt the Supreme Court to rethink the way the justices analyze First Amendment cases. For years, the court has provided maximum protection to political, educational, and artistic speech. But the justices have accorded a lower level of protection for commercial speech, such as paid advertisements by a business attempting to sell products or services.

There are signs that the court may be prepared to change its dual approach in First Amendment cases, with five justices having expressed reservations about treating commercial speech differently from other forms of protected speech.

The most forceful expression came in a 1996 concurring opinion, in which Justice Clarence Thomas wrote: "I do not see a philosophical or historical basis for asserting that 'commercial' speech is of 'lower value' than 'noncommercial speech.' Indeed, some historical materials suggest to the contrary."

Floyd Abrams, one of the nation's leading First Amendment lawyers, filed a friend-of-the-court brief on behalf of billboard companies that echoes Justice Thomas's concerns.

"We urge the court ... to treat truthful, nonmisleading commercial speech the same as noncommercial speech - and to apply [the toughest level of protections] to content-based, paternalistic regulations designed to protect the public from the perceived deleterious effects of such speech," Mr. Abrams writes.

Free-speech advocates say there is rarely, if ever, a justification for the government to substitute its judgment for that of individual citizens in determining what information should or should not be disclosed to the public.

Antitobacco groups counter that the Massachusetts restrictions aren't related to the content of the ads, only their location in areas frequented by children. In addition, they say that when children are endangered by enticing advertisements, restrictions are appropriate.

"When young children are constantly and involuntarily bombarded with seductive messages appearing on neighborhood storefronts that promote a lifelong addiction, not only is their health endangered, but their right to be free from having adult choices foisted upon them is not so subtly infringed," writes Donald Garner in a brief on behalf of the American Medical Association and other medical groups.

"When parental admonitions against smoking are trivialized by in-store cigarette displays at eye level next to the candy, the right of parents to guide their children toward a life of health and vitality is undermined," Mr. Garner adds.

Others warn that the Massachusetts regulations might lead to creation of a "child protection" exception to the First Amendment, opening the door to broad and unchecked censorship.

Writes Steven Brody in a friend of the court brief on behalf of the Association of National Advertisers: "While efforts to childproof the flow of speech in our society might be politically popular, such restrictions would impose a constitutionally intolerable burden on adult-to-adult communications and would intrude on the rights of parents to control the education of their children."